1. Field of the Invention
The present invention relates to a field of optical display systems. More particularly, the present invention relates to a real image projection device improved by using a low-cost plastic curved mirror and using several combinations of methods for reducing aberrations, so as to reduce ghost and astigmatism phenomenon.
2. Description of Related Art
The present invention pertains to a real image projection system, and in particular, to a system in which an image of a real object is formed in space, giving the illusion that a real object exists at that point in space, when in reality it does not. Real image projection systems normally incorporate spherical or parabolic mirrors for imaging. In large systems, where the viewer is located at a significant distance from the image being viewed, optical aberrations, such as spherical aberrations and astigmatism in particular, are not as much of a problem as in small systems where the viewer is located close to the image. Astigmatism causes eye strain when viewing the image for a long period of time, and this has been one of the primary reasons that small real image projection systems have not been widely incorporated in gaming applications as well as in workstation applications.
Another reason for the lack of wide-spread acceptance of the small real imaging systems is that ghost images in the systems are much more noticeable when viewing the display from a close distance. Many approaches have been used to reduce ghosting, including tinted beamsplitters and circular polarizers, none of which are extremely effective. Even with the use of a circular polarizing window, the ghost images are visible even though they can be significantly reduced. The circular polarizing windows typically have a maximum transmission of 42%, which significantly reduces image brightness. Thus, in an arcade or other public area that is brightly lit, the real image is usually difficult to see.
Other optical aberrations present problems for the real image projection systems. For example, field curvature distortion is a significant problem for the small systems because of the shorter focal lengths typically associated therewith. Particularly, a rectangular shape displayed on a CRT screen projects as a “fish-eyed” real image of the target object. The sides of the rectangular image appear to bow outward and the center of the rectangular image appears magnified, as compared to the edges. This is a natural phenomenon of spherical mirrors, and cannot normally be corrected without a significant number of additional lenses in the beampath, which significantly increases a size of the display system as well as the cost of manufacturing such displays.
Optics has been designed to compensate for some of these aberrations, such as spherical aberrations, through use of a Mangin mirror that has a reflective convex spherical surface with a longer radius, and a transmissive concave spherical surface with a shorter radius. However, this approach is not practical for the real image projection system, because the image source or the target is not a point at the focal point or center of curvature of the mirror, as in a single point imaging system. In the real image projection system, the target is usually a rectangle, such as a monitor screen, where only the center of the screen is on the axis or at the focal point of the mirror. The Mangin dual curve corrective mirror could be significantly improved by replacing the concave spherical surface with an aspheric surface of revolution, which will reduce the astigmatism for points offset from the axis of the mirror. Thus, the Mangin mirror incorporating two spherical curves is extremely effective for points along the axis of an on-axis system, but the problem of astigmatism becomes progressively worse as the target point deviates from the axis of the mirror curvature. An aspheric curve on the concave surface would optimize the correction and reduce the astigmatism for a large area around the axis or focal point.
Another reason that the small systems have not become a mainstream is because of the difficulty in producing curved optical apparatuses in reasonable amount. The problem is compounded when corrective optical curvatures are incorporated.